Switch to `HttpClient`

[Zastai]

This switches both OAuth2 and Query to use HttpClient (provided by new dependency System.Net.Http) instead of the deprecated WebClient.

This brings with it a slew of breaking API changes:

• In both OAuth2 and Query:
  • Changes to constructor signatures
  • New: an HttpClient instance can be provided
    • The ownership of that client can stay with the caller, in which case that is the only client that will be used, and Close() throws an exception.
    • Otherwise, an HttpClient instance will be created whenever needed, and Close() can be used to ensure any such client is cleaned up (with the next request creating a new one). Both the creation and further configuration of such clients can be done via user code.
  • Because of this use of a externally-created clients, request behaviour has been adjusted. They will:
    • use absolute URIs constructed using the UrlScheme, Server and Port properties (so any base address set on the client will be ignored)
    • use application/json for the Accept header (so any default value configured on the client for that header is ignored)
    • add the library name and version at the end of the UserAgent header (so any default value configured on the client for that header is honored)
  • The [Default]WebSite property has been renamed to [Default]Server.
• In Query only:
  • DefaultUserAgent and UserAgent are now a list of header values instead of a single string
  • UserAgentUrl was dropped
• Elsewhere:
  • QueryException now has two additional fields (Code and Reason), with new and change constructor signatures